Organic light emitting diodes (OLEDs) typically include several layers of organic materials interposed between conductive thin film electrodes, with at least one of the organic layers being an electroluminescent layer. When a voltage is applied to electrodes, holes and electrons are injected by an anode and a cathode, respectively. The holes and electrons injected by the electrodes migrate through the organic layers to reach the electroluminescent layer. When a hole and an electron are in close proximity, they are attracted to each other due to a Coulomb force. The hole and electron may then combine to form a bound state referred to as an exciton. An exciton may decay through a radiative recombination process, in which a photon is released. Alternatively, an exciton may decay through a non-radiative recombination process, in which no photon is released. It is noted that, as used herein, internal quantum efficiency (IQE) will be understood to be a proportion of all electron-hole pairs generated in a device which decay through a radiative recombination process.
A radiative recombination process can occur as a fluorescence or phosphorescence process, depending on a spin state of an electron-hole pair (namely, an exciton). Specifically, the exciton formed by the electron-hole pair may be characterized as having a singlet or triplet spin state. Generally, radiative decay of a singlet exciton results in fluorescence, whereas radiative decay of a triplet exciton results in phosphorescence.
More recently, other light emission mechanisms for OLEDs have been proposed and investigated, including thermally activated delayed fluorescence (TADF). Briefly, TADF emission occurs through a conversion of triplet excitons into singlet excitons via a reverse inter system crossing process with the aid of thermal energy, followed by radiative decay of the singlet excitons.
An external quantum efficiency (EQE) of an OLED device may refer to a ratio of charge carriers provided to the OLED device relative to a number of photons emitted by the device. For example, an EQE of 100% indicates that one photon is emitted for each electron that is injected into the device. As will be appreciated, an EQE of a device is generally substantially lower than an IQE of the device. The difference between the EQE and the IQE can generally be attributed to a number of factors such as absorption and reflection of light caused by various components of the device.
An OLED device can typically be classified as being either a “bottom-emission” or “top-emission” device, depending on a relative direction in which light is emitted from the device. In a bottom-emission device, light generated as a result of a radiative recombination process is emitted in a direction towards a base substrate of the device, whereas, in a top-emission device, light is emitted in a direction away from the base substrate. For example, in a bottom-emission device, an electrode that is proximal to the base substrate is generally made to be light transmissive (e.g., substantially transparent or semi-transparent), and an electrode that is distal to the base substrate is generally made to be reflective.
Depending on the specific device structure, either an anode or a cathode may act as a reflective electrode in a bottom-emission device. In a typical bottom-emission device, however, the reflective electrode is generally chosen to be the cathode. Materials which are typically used to form the reflective cathode include metals such as aluminum (Al), silver (Ag), and various metallic alloys.
Since various portions of the OLED devices are easily degraded when exposed to reactive materials such as oxygen and moisture which are present in the air, they are typically encapsulated to inhibit such reactive materials from contacting the device. For example, OLED devices may be encapsulated using thin-film encapsulation (TFE), a combination of a barrier film and barrier adhesive, or combination thereof.
However, encapsulation of OLED devices remains a challenge for a number of reasons, such as due to the relatively high permeability of the materials used for encapsulation, and/or difficulties associated with forming and maintaining a hermetic seal between the encapsulant and the surface of the OLED device. Particularly with respect to flexible OLED devices, many encapsulant or barrier solutions do not possess suitable mechanical properties for allowing the encapsulated OLED device to remain flexible while providing sufficient barrier properties. For example, some encapsulants contain brittle and/or inelastic materials which break or deform when bent, thus rendering the OLED device unusable. Some materials used to form barrier layers and barrier adhesives are also known to exhibit relatively high water vapor transmission rate (WVTR), and thus may be undesirable for use in devices which require high environmental stability. While barrier films exhibiting relatively low WVTR of less than 10−6 g/m2·day also exist, such films are generally expensive.
In addition to the above, a number of OLED encapsulation solutions or processes require additional equipment, such as deposition chamber(s), coaters and/or laminators for performing the encapsulation of the device. Moreover, such processes typically require an unencapsulated OLED device to be transferred to the encapsulation equipment, subsequent to the deposition of the top electrode (e.g. a cathode) for completing the fabrication of the OLED device. In some cases, the unencapsulated OLED device may become exposed to air or other reactive gases during the transfer or during the encapsulation stage, which can lead to degradation and/or contamination of the device even before the device has been encapsulated.